2.0 Analysis 2.1 Main Hydraulic System - Loss of Pressure The main hydraulic system fluid loss was traced to a leak in the right main landing gear maxaret return line. The failure of the line led to a complete evacuation of the main system hydraulic fluid and the subsequent loss of main system pressure. The line failure was traced to a circumferential split of the inner tube core, which was likely caused by an overdeflection of the line by some external source, rather than by normal service flexing. The location of the line, along and just behind the main landing gear strut, is such that damage could occur from contact made with equipment during jacking of the main wheel or during removal of the wheel brake units; however, there is no evidence that damage occurred during either of these procedures. 2.2 Brake System 2.2.1 No. 3 Maxaret Unit Testing During the post-occurrence examination, the maxaret return line was replaced and the brake system was functionally checked. The no. 3 maxaret unit failed the wheel spin-up test, but passed the bench-test. As no faults could be found in the maxaret unit that would have contributed to a loss of brake pressure, it is likely that the maxaret unit failed the wheel spin-up test because of air trapped in the hydraulic system as a result of the maxaret return line failure. 2.2.2 Brake System Pressure The main hydraulic system had failed as a result of fluid leaking from the maxaret return line located on the return side of the brake system, which would not have affected brake system hydraulic pressure and operation. The crew reported that, after the main hydraulic system failure occurred, the brake system pressure remained steady at 2,000 psi for nearly 1 hours prior to the landing, which indicates that the NRVs were effectively isolating the brake system from the main hydraulic system, and that some fluid was stored in the brake accumulators for braking during landing. The brake accumulators are considered fully discharged at 1,300 psi. Therefore, at a brake system pressure of 2,000 psi, there was only 700 psi of hydraulic pressure in the accumulators available for brake operation, which would have allowed much fewer than the approximately nine moderate brake applications normally available when the accumulators are fully charged at 2,500 psi. With a high aircraft landing weight and only about five moderate brake applications available at best, any further drop in brake pressure would have jeopardized the stopping ability of the aircraft. 2.3 Brake System - Loss of Pressure 2.3.1 Introduction It was not determined why the brake system lost its pressure and failed to provide sufficient braking during the landing roll. The captain reported that, when he applied the brakes after touchdown, no braking action was apparent and brake pressure was lost. However, no faults were found that could explain the brake system failure during the landing roll. The following possible scenarios were explored and may offer some explanation for the loss of brake pressure. 2.3.2 Cold Weather Operations Over the previous few winter months, the aircraft had experienced a history of cold-weather-related hydraulic system problems. The weather was cold during the flight, and its effect on the internal seals may have resulted in some internal leakage of fluid through either the brake system pressure reducing valves, the brake control valve, or a maxaret anti-skid unit. Although a stuck or unseated valve in any one of these components could account for a rapid loss of pressure (since fluid would be returned to the main hydraulic system reservoir), no such system faults were found when the aircraft was pulled into a warm hangar for examination. 2.3.3 Inadvertent Activation of Brakes Inadvertent, partial application of the brakes by the crew during operation of the rudder in flight, or during the approach and landing, would have decreased the operating brake pressure. However, such a possibility is unlikely in light of the crew's awareness of the main hydraulic system malfunction and the consequences of unnecessary brake activation on available brake pressure. 2.3.4 Activation of Wheel Brake Maxaret Anti-skid Unit Forty-five minutes prior to landing, the condition of the runway was reported as being 90 per cent frost covered with 10 per cent compacted snow patches and a JBI reading of 0.420. The AIP indicates that, with a JBI reading of 0.420, the normal stopping distance with wheel brakes only could be increased by as much as 45 per cent. Such conditions may have resulted in wheel skid during the landing roll. Activation of the maxaret anti-skid unit, as a result of either rapid application of the brakes or the runway conditions being conducive to wheel skid, would have resulted in a loss of brake pressure through the maxaret units. However, the crew initially used the drag created by the propellers to decelerate the aircraft, and reported that they applied the brakes slowly to avoid the operation of the maxaret anti-skid units. 2.4 Runway Excursion At the time of the occurrence, the observed surface winds were from 270 to 300 degrees at 10 knots gusting to 15. Given the runway heading of 234 degrees, the winds would have constituted a right cross-wind component, blowing between 35 and 65 degrees off the runway heading. The crew had turned the nosewheel steering off prior to landing to conserve hydraulic pressure, and were relying on the rudder and differential braking for directional control during the landing roll. Because there was no brake pressure, differential braking was not available for directional control. As the aircraft slowed down and the rudder became ineffective, directional control was progressively lost, and the aircraft began to weathercock into the prevailing cross-wind. The aircraft gradually veered to the right and exited the runway. 2.5 Flight Crew/Cabin Crew Coordination and Communication The flight crew communicated early with company operational control in order to confirm their best option. In preparation for the landing, the flight crew advised the FSS of the situation and requested ERS on standby. The flight crew communicated early with the cabin crew and maintained good communication. Passengers were advised of the situation and briefed early in preparation for evacuation after the landing. The good coordination and communication between the flight crew and the cabin crew in preparation for the landing was instrumental in the successful evacuation of the occupants following the runway excursion. 3.0 Conclusions 3.1 Findings The main hydraulic system fluid loss was traced to a leak in the right main landing gear maxaret (anti-skid) return line. The failure of the maxaret return line led to a complete evacuation of the main hydraulic system fluid and the subsequent loss of main system pressure. The line failure consisted of a circumferential split of the inner tube core, which was likely the result of an overdeflection of the line caused by some external loading rather than normal service flexing. The source of the overdeflection could not be confirmed. The crew and the company evaluated the situation and elected to continue to Thompson. It could not be determined why the brake system lost its pressure and failed to provide sufficient braking during the landing roll. As the aircraft slowed down, directional control was progressively lost due to the rudder becoming ineffective and differential braking not being available. The aircraft gradually weathercocked into the prevailing right cross-wind and exited the runway. The good coordination and communication between the flight crew and cabin crew in preparation for the landing were instrumental in the successful evacuation of the occupants following the runway excursion. 3.2 Causes The main hydraulic pressure was lost due to a leak in the right maxaret (anti-skid) return line, which failed due to overdeflection from an undetermined source. Directional control was lost during the landing roll due to a loss of brake pressure, the cause of which was not determined. As the aircraft slowed down, it weathercocked into the prevailing cross-wind and departed the runway. The Board has no aviation safety recommendations to issue at this time.4.0 Safety Action The Board has no aviation safety recommendations to issue at this time.